In the monitoring device known from WO 00/52243 the observation interval is adjusted at the microprocessor of the control unit of the weft thread detector. Since the point along the thread path at which the weft thread is released can be adjusted by the weaver in dependence upon certain factors, such as the kind of cloth, the cloth width, or the like, also the observation interval must be readjusted accordingly so as to maintain the precision of monitoring. This is complicated.
In weaving machines so-called arrival sensors are frequently used, cf. e.g. EP 0 374 398 A, which output a signal when the weft thread arrives at the cloth edge. These arrival sensors are mostly opto-electronic sensors whose sensitivity must be adjusted to the thread quality in each individual case, whose function is impaired by unavoidable lint, and, when the weaving machine is changed over, it may become necessary to re-position these sensors. Also this procedure is complicated.
It is the object of the present invention to provide a monitoring device of the type mentioned at the beginning, in which the observation interval will be terminated precisely in a structurally simple manner and independently of the thread quality and of possible adjustments of the release time at the weaving machine, e.g. for simplifying the sensitivity adjustment of a self-adaptive weft thread detector.
Surprisingly enough, the aim of terminating the observation interval precisely and exactly at the right point of the thread path can be achieved by a knack, viz. that the weft thread is not scanned directly but that an electronic sensor is used for monitoring the arrival of the thread clip at the release position. The thread quality is of no importance in this connection. Changeovers in the weaving machine, which alter the moment of release along the thread path, do not influence the precision. It goes without saying that the sensor will not inevitably respond to the release position reached by the thread clip, but, if necessary, it will already respond to the movement of the thread clip to the release position. In a weft thread detector with automatic adjustment of the sensitivity to the respective optimum over a plurality of successive insertions, sensitivity adaptation will be facilitated by this sensor which controls the end of the observation interval and which responds to the arrival of the thread clip at the release position. When the sensor indicates that the thread clip has reached the release position, it operates with a consciously accepted uncertainty factor, since its signal does not represent reliably whether or not the weft thread has arrived at this point. This is, however, irrelevant, since the adequate movement of the weft thread up to the end of the observation interval is monitored by the weft thread detector itself in any case. Any sensor, e.g. an opto-electronic sensor, an approximation sensor, a piezo-electric sensor or the like, which is capable of detecting the moment at which the thread clip reaches its release position, is suitable to be used as an electronic sensor.
Since in rapier weaving machines or projectile weaving machines the thread clip is predominantly moved to the release position by a collision impact, and since substantial kinetic energy is exchanged at the moment of this collision impact, a piezo sensor which is acted upon by this energy shock and which outputs the signal in response to a transmission of said collision impact will be particularly suitable for use as a sensor.
A high operational reliability and an expressive signal are obtained when the sensor is arranged directly on the stop element in the opening device, the opening device and/or the stop element being adjustable relative to the path of movement of the insertion element. This position of the sensor guarantees that the sensor will directly scan the collision impact when the thread clip reaches the release position and that, if changeovers are necessary in the weaving machine in the area of the opening device, it will take part in these changeovers so that readjustments will not be necessary at this sensor, neither in the case of changes in the thread quality nor in the case of such changeovers.
The sensor can comprise a fork-shaped body with at least one piezo-ceramic element in the area of a fork prong, said body being, if desired, attached to the stop element or inserted therein and secured in position.
If an opening device having a control plate as a stop element is used, the sensor should be secured in position on or in said control plate.
If the sensor in question is a piezo sensor, which responds to the collision impact, it will be expedient to arrange the sensor in the vicinity of the opening surface of the plate at a point where the collision impact is discernible clearly and in an unadulterated form.
The sensor could be completed by providing in the body or in the signal transmission path an amplifying component for signal conditioning. The signal outputted and conditioned by the sensor can be directly used for terminating the observation interval in this way.
Finally, it may be expedient when, in the case of comparatively small cloth widths, the sensor is connected via a cable to the weft thread detector, or to the control of said weft thread detector, or to the control of the weaving machine. In the case of comparatively large cloth widths, it is imaginable to choose a wireless signal transmission because the distance can then be comparatively large.